The reaction $A \rightarrow B$ is carried out isothermally on a porous catalyst. The intrinsic reaction rate is $kC_{A}^{2}$, where $k$ is the rate constant and $C_{A}$ is the concentration of $A$. If the reaction is strongly pore-diffusion controlled, the observed order of the reaction is

1. $1$
2. $2$
3. $\frac{3}{2}$
4. $\sqrt{2}$